Looks like you have 95% of your answer already - just draw in the screen, extend a line from the lens to the bottom of the screen and see what clearance it gives you over the seats.....or am I missing something? On very rough examination it looks like it will be tight - maybe giving 12" or so above the rear of each seat cushion???
Also, from your sketch it looks like you're planning on mounting the projector tight against the rear wall - you'll need to allow some space behind the PJ for cooling & connections, so the reduced distance from the screen will need to be taken into consideration. If interference with the light path is going to be close, you obviously need to be as accurate as possible in your drawing. If you can still get the desired image size, moving the projector closer to the screen (or the screen away from the PJ!) will win you a little more headroom above the seats.